High resistance steel band or sheet and method for the production thereof

ABSTRACT

The invention relates to a higher-strength steel strip or steel sheet comprising a predominantly ferritic-martensitic microstructure with a martensite content of between 4 and 20%, wherein the steel strip or steel sheet, apart from Fe and impurities due to smelting, comprises (in % by weight) 0.05-0.2% C, ≦1.0% Si, 0.8-2.0% Mn, ≦0.1% P, ≦0.015% S, 0.02-0.4% Al, ≦0.005% N, 0.25-1.0% Cr, 0.002-0.01% B. Preferably the martensite content is approximately 5% to 20% of the predominantly martensitic-ferritic microstructure. Such a higher-strength steel strip or steel sheet made from a dual phase steel comprises good mechanical/technological properties even after being subjected to an annealing process which includes an overageing treatment. Furthermore, the invention relates to a method for producing steel strip or steel sheet according to the invention.

BACKGROUND OF THE INVENTION

The invention relates to a higher-strength steel strip or steel sheetcomprising a predominantly ferritic-martensitic microstructure, as wellas to a method for its production.

Within the context of the use of steel strip and steel sheet of the typementioned above, there are increasingly demanding requirements inrespect to their versatility, useability and service properties. Thus,continually improved mechanical characteristics of such steel strip andsteel sheet are demanded. This relates in particular to the formingproperties of such materials.

A steel strip or steel sheet with good forming properties ischaracterised by high r-values which represent good deep drawingproperties, high n-values which represent good stretch formingproperties, and high strain values which indicate positive plane-strainproperties. A low yield strength ratio, calculated from the ratio ofyield strength and tensile strength, is also characteristic of goodstretch forming properties.

The general requirement for increased strength includes increasedefforts in the area of lightweight construction. In this field, sheetsof reduced thicknesses are used so as to save weight. The loss ofstrength which is associated with lightweight design, can be compensatedfor by an increase in the strength of the sheet itself. However, anyincrease in strength naturally results in a decrease in formingproperties. It is thus the prime objective of further improvements inmaterials of the type discussed in this instance, to increase thestrength while at the same time keep the decrease in forming propertiesas low as possible.

The steel-iron materials sheets 093 and 094 list numeroushigher-strength micro-alloyed or P-alloyed steels with good coldformability. Some of these steels have bake-hardening characteristics.These characteristics can in particular be achieved by applying acontinuous annealing process which if needed is linked with a hot diprefining process.

In addition, in practical application successful attempts have been madeto increase the strength of steels while at the same time achievingsignificantly better forming properties, by increasing the alloycontents. By way of a supplement or an alternative, it has been possibleto improve these characteristics with higher cooling rates during thehot roll process or the continuous annealing process. However, thisapproach is associated with a disadvantage in that the increasedcontents of alloying elements and the installation and operation of therequired cooling equipment result in increased costs.

Conventional continuous annealing plants for sheet comprise anoverageing furnace behind the annealing and cooling parts. In such anoverageing zone, “overageing” of the steel strip or steel sheet occursin that the processed steel strip or steel sheet is kept within atemperature range of ≦500° C. In the case of low-alloyed, soft steels,such holding at a temperature of up to 500° C. causes extensiveprecipitation of dissolved carbon as carbide. As a result of thisprecipitation of carbide, the mechanical/technological properties of thesteel strip or steel sheet are positively influenced. However, in theproduction of dual phase steels in continuous annealing plants,undesirable tempering effects in the martensite can occur during thepassage through the overageing zone.

SUMMARY OF THE INVENTION

It is thus the object of the invention to create a higher-strength steelstrip or steel sheet made from a dual-phase steel, said steel strip orsteel sheet comprising good mechanical/technological properties evenafter being subjected to an annealing process which includes anoverageing treatment. Furthermore, a method for producing such strip orsheet is to be disclosed.

This object is met by a higher-strength steel strip or steel sheetcomprising a predominantly ferritic-martensitic microstructure with amartensite content of between 4 and 20%, wherein the steel strip orsteel sheet, apart from Fe and impurities due to smelting, comprises (in% by weight) 0.05-0.2% C, ≦1.0% Si, 0.8-2.0% Mn, ≦0.1% P, ≦0.015% S,0.02-0.4% Al, ≦0.005% N, 0.25-1.0% Cr, 0.002-0.01% B. Preferably themartensite content is approximately 5% to 20% of the predominantlymartensitic-ferritic microstructure.

A steel strip or steel sheet according to the invention features highstrength of at least 500 N/mm² while at the same time featuring goodforming properties, without there being a need for particularly highcontents of particular alloying elements. In order to increase strength,the invention makes use of the transformation-influencing effect of theelement boron, such effect being already known per se in the case ofsteels for hot rolled strip and forged pieces. In this, thestrength-increasing effect of boron is ensured in that according to theinvention at least one alternative nitride former, preferably Al and asa supplement Ti, is added to the steel material. The effect of addingtitanium and aluminium consists of their binding the nitrogen present inthe steel, so that boron is available to form hardness-increasingcarbides. Supported by the necessarily present Cr content, in this way ahigher level of strength is achieved when compared to comparative steelsof conventional compositions.

As mentioned, the strength-increasing effect of boron in steels hasalready been discussed in the state of the art in the context ofproducing hot strip or forged pieces. Thus the German publishedapplication DE 197 19 546 A1 describes for example a hot strip of thehighest strength, with optionally Ti being added by alloying, to saidhot strip, in a quantity which is sufficient for a stoichiometricalfixation of the nitrogen present in the steel. In this way, the quantityof boron which has also been added, is protected against fixation tonitrogen. The boron can thus contribute without hindrance to increasingthe strength and the through-hardenability of the steel. Furthermore,the German published application DE 30 07 560 A1 describes theproduction of a higher-strength hot-rolled dual-phase steel to whichboron in quantities of 0.0005 to 0.01 weight % is added. In this case,boron is added to delay the ferrite-pearlite transformation.

Surprisingly, it has been shown that in the case of a higher-strengthsteel strip or steel sheet according to the invention, the quantity ofmartensite remains, even if after cold rolling, the respective materialis subjected to an annealing treatment with subsequent cooling andoverageing or if it is subjected to a hot dip refining process. Theyield strengths of a strip or sheet according to the invention arebetween 250 N/mm² and 350 N/mm². The tensile strengths are 500 N/mm² tomore than 600 N/mm², in particular up to 650 N/mm². In the non-dressedstate, the material is practically free of yield strength elongation(A_(RE)≦1.0). A steel strip or steel sheet according to the inventionthus comprises properties and characteristics which it was hitherto notpossible to achieve in the case of low-alloyed steels.

A further advantage of steels according to the invention, is theirresistance to tempering effects. The presence of chromium in steelsaccording to the invention, prevents the problem which in particularoccurs in the case of dual-phase steels of conventional composition,namely the problem that the martensite content is tempered duringoverageing treatment and that in this way a decrease in strength occurs.

Preferably a steel strip or steel sheet according to the inventionadditionally comprises a Ti content of at least 2.8×A_(N), whereinA_(N)=content of N in % by weight. In this, the Al content can belimited to a range of 0.02-0.05% by weight. In this embodiment of theinvention, the nitrogen contained in the steel is offered Al as anitride former and in addition there is also a quantity of Ti presentwhich is sufficient for the stoichiometrical nitrogen fixation. Bycontrast, if no Ti is present in the steel, the Al content of the steelstrip or steel sheet should range between 0.1 to 0.4% by weight. Due tothe presence of aluminium and/or titanium, first of all relativelylarge-grain TiN and/or AlN form(s) during cooling. Since titanium andaluminium have a greater affinity to nitrogen than does boron, theexisting boron content is available for carbide formation. This has amore favourable effect on the mechanical properties of steels accordingto the invention, than is the case where, in the absence of adequatecontents of titanium or aluminium, for example at first small-grained BNis precipitated.

One option of producing steel strip or steel sheet according to theinvention consists of producing the steel strip or steel sheet by coldrolling a hot strip. As an alternative, it is however also possible toprocess a thin hot strip without further cold rolling to produce a steelstrip according to the invention, provided its thickness is sufficientlyreduced for further processing. Such a hot strip can for example beproduced on a direct strand reduction mill in which a cast steel strandis directly rolled to a thin hot strip. Irrespective as to which methodof producing the steel strip or steel sheet is selected, theabove-mentioned object concerning the production method is met in thatthe steel strip or steel sheet is subjected to an annealing treatment inthe continuous furnace during which treatment the annealing temperatureis between 750° C. and 870° C., preferably between 750° C. and 850° C.,and in that the annealed steel strip or steel sheet is subsequentlycooled down from the annealing temperature at a cooling rate of at least20° C./s and at most 100° C./s.

With the process according to the invention, based on a C—Mn steel towhich boron and at least Al and if need be by way of a supplement Tihave been added as a nitride former, a steel strip can be produced thateven at the annealing and cooling conditions stated, comprises thedesired high martensite content of approximately 5% to 20%. Contrary tothe conventional approach, this does not require the steel strip orsteel sheet, after continuous annealing, to be cooled at a high coolingrate, so as to form martensite in the microstructure. Instead, theboron, which is freely dissolved in the lattice, ensures that martensiteformation occurs even at low cooling rates such that a predominantferrite/martensite microstructure with the property combinations whichare typical for dual-phases, results. It has been found that this effectis already effective at a boron content of 0.002 to 0.005%. Thus theinvention makes it possible to produce a higher-strength steel strip orsteel sheet without the need for expensive devices for cooling orwithout the use of large quantities of alloying elements.

Furthermore it has been found that steels produced according to theinvention do not experience any degradation worth mentioning, in theirproperties, as a result of tempering effects in the martensite, whenundergoing overageing. In those cases where no hot dip refining of thesteel strip or steel sheet is carried out, overageing can last up to 300s at a treatment temperature between 300° C. and 400° C. By contrast, ifhot dip refining, for example hot galvanising, does take place, then theholding period during possible overageing during galvanising should lastup to 80 s, with the treatment temperature being between 420° C. and480° C. Furthermore, the properties of a galvanised steel strip or steelsheet produced according to the invention can be further improved inthat after galvanising, galvannealing treatment which is know per se, iscarried out. During such treatment, hot galvanised sheet or strip isannealed after hot dipping. Depending on the particular application, itmay moreover be advantageous if the steel strip or steel sheet issubsequently dressed.

DETAILED DESCRIPTION OF THE INVENTION

Below, the invention is explained in more detail with reference toembodiments.

Table 1 shows the alloying contents and the technological/mechanicalcharacteristic values A_(RE) (yield strength elongation), R_(eL) (loweryield strength), R_(m) (tensile strength), R_(el)/R_(m) (yield strengthratio) and A₈₀ (elongation to fracture) for steel strip A1-A4 accordingto the invention. By way of comparison, the same table shows therespective information for comparison steel strip B1-B5, C1-C5, D1-D4and E1.

In the case of all steel strip A1-E1 according to the invention, shownin Table 1, said steel strip being shown for comparison, the C contentis between 0.07 and 0.08% by weight. In the case of the shown comparisonsteel strip B1-B5, the Mn content of 1.5-2.4% by weight has been used toinfluence the transformation behaviour. In the case of the comparisonsteel strip C1-C5, for the same purpose an element combination of Si(around 0.4% by weight) and Mn (1.5-2.4% by weight) and in the case ofthe comparison steel strip D1-D4 a combination of the contents of Si (upto 0.7% by weight), Mn (1.2-1.6% by weight) and Cr (0.5% by weight) havebeen used. In the case of the comparison steel strip E1, Mo has beenprovided in addition.

In the case of the steel strip A1-A4 according to the invention, apartfrom Si (up to 1.0% by weight) and Mn (0.8-1.5% by weight) which havealso been used, the highly transformation-delaying property of boron hasbeen taken advantage of. To prevent the formation of boron nitrides, thenitrogen was fixed with Ti as a nitride former. The Ti content presentfor this purpose was around 0.03% by weight in the case of N contents of0.004 to 0.005% by weight, while the B content was approx. 0.003% byweight.

After smelting the steels A1-A4 and pouring a slab of each at a time,the respective slab was heated to 1170° C. Each heated slab was thenrolled to form a hot strip with a thickness of 4.2 mm. The finishingrolling temperature ranged between 845 and 860° C. Subsequently, the hotstrip was coiled at a temperature of 620° C., with the average coilcooling being 0.5° C./min. Subsequently the hot strip was pickled andcold rolled to a thickness of 1.25 mm.

The respective cold-rolled steel strip was subjected to a continuousannealing process which was guided by a standard furnace practice withoverageing for low-alloyed soft steels. An annealing temperature duringcontinuous annealing of 800° C. and a two-step cooling with finalpassing through the overageing zone were essential characteristics ofthis annealing and overageing treatment. At first, cooling was down to550-600° C. at a cooling rate of approx. 20° C./s. Subsequently, coolingtook place at a cooling rate of approx. 50° C./s to 400° C. Thesubsequent overageing treatment consisted of holding the strip at atemperature range of 400-300° C. for a period of 150 s.

The mechanical/technological characteristic values shown in Table 1 forthe steel strip A1 to A4 produced according to the invention, afterconventional continuous annealing in the non-dressed state, document theadvantageous properties of the steel strip or steel sheet producedaccording to the invention, when compared to the additionally shownhigher-strength alloying concepts of the comparison steel strip. Thefact that in the case of the steel strip according to the inventionthere is no yield strength elongation in the non-dressed state, clearlypoints to the favourable ferrite/martensite microstructure formation.The elongation limits are below 300 N/mm² and the strength valuesbetween 530 N/mm² and 630 N/mm². Thus the respective steel strip A1-A4exhibits good hardening behaviour during plastic deformation. This alsomanifests itself in a very low yield strength ratio (R_(e)/R_(m)<0.5).In the case of strengths of 540-580 N/mm² the elongation at fracture isbetween 27 and 30%; in the case of approx. 630 N/mm² it is still a good25%. On the whole, the mechanical properties are isotropic.

In a predominant number of cases, all the comparison steel strip withstrengths at the level of steel strip according to the invention,exhibit less favourable strain values, above all at significantlyincreased values of yield strength elongation. This expresses moreunfavourable hardening behaviour.

In the case of comparison steel strip a lack of yield strengthelongation can only be achieved by very high Mn contents of more than2.1% by weight (comparison steel strip B4, B5, C5). Furthermore,significantly higher strength values are found. At the same timehowever, less favourable yield strength ratios and smaller elongationsare achieved.

Table 2 shows the alloying contents and the technological/mechanicalcharacteristic values A_(RE) (yield strength elongation), R_(eL) (loweryield strength), R_(m) (tensile strength), R_(eL)/R_(m) (yield strengthratio) and A₈₀ (elongation to fracture) for steel strip F1 according tothe invention. To produce the steel strip F1, first a Ti—B alloyed C—Mnsteel was smelted and then hot rolled and cold rolled in theconventional way. Subsequently the cold-rolled steel strip F1 wasannealed and conveyed through a hot galvanising plant.

Annealing was carried out at 870° C. This was followed by a holdingphase of 60 seconds at 480° C. The temperature of the galvanising zincbath was 460° C. Table 3 shows the details of the operating conditions.The properties of the steel strip F1 which was hot-dip refined in thisway and subsequently dressed, are within the range of the properties ofthe strip according to the invention, whose values appear in Table 1.

Table 4 shows the alloying contents and the technological/mechanicalcharacteristic values A_(RE) (yield strength elongation), R_(eL) (loweryield strength), R_(m) (tensile strength), R_(eL)/R_(m) (yield strengthratio) and A₈₀ (elongation to fracture) for steel strip G1¹-G1⁴according to the invention. Each of the steel strip G1¹-G1⁴ was producedbased on a steel of identical composition and was subjected to aconventional hot rolling and cold rolling process.

The cold rolled steel strip G1¹ and G1² were subjected to continuousannealing treatment while the steel strip G1³ and G1⁴ were subjected tohot galvanising treatment. Table 5 shows the respective operationalconditions. With annealing temperatures of 780-800° C., the tensilestrengths of the steel strip G1¹-G1⁴ are around 500 N/mm². Commencementof creeping is largely free of yield strength elongation (A_(RE)≦1.0%).

TABLE 1 Steel C Si Mn P S Al N Cr Mo Ti B A_(Re) R_(eL) R_(m) R_(eL)/RA₈₀ strip [% by weight] [%] [N/mm²] [N/mm²] [—] [%] A1 0.08 0.01 1.480.01  0.012 0.04 0.004 0.5  — 0.028 0.003  0 258 544 0.47 27 A2 0.080.39 1.23 0.01  0.012 0.03 0.004 0.5  — 0.028 0.0032 0 252 531 0.47 30A3 0.08 0.79 1.24 0.009 0.012 0.03 0.005 0.51 — 0.029 0.0032 0 260 5820.45 28 A4 0.08 0.78 1.46 0.009 0.013 0.04 0.004 0.51 — 0.029 0.003  0266 631 0.42 25 B1 0.07 0.01 1.53 0.012 0.01  0.03 0.005 — — — — 3.6 366475 0.77 24 B2 0.07 0.03 1.87 0.011 0.013 0.02 0.004 — — — — 1.2 350 5570.63 17 B3 0.07 0.01 1.95 0.011 0.01  0.03 0.004 — — — — 1.0 350 6020.58 15 B4 0.08 0.02 2.14 0.012 0.009 0.03 0.003 — — — — 0 389 701 0.5515 B5 0.08 0.03 2.4  0.011 0.011 0.04 0.004 — — — — 0 522 852 0.61 11 C10.08 0.42 1.53 0.019 0.012 0.03 0.005 — — — — 3.6 428 571 0.75 30 C20.07 0.38 1.63 0.011 0.011 0.03 0.003 — — — — 3.0 420 583 0.72 28 C30.08 0.35 1.93 0.012 0.013 0.03 0.004 — — — — 1.2 407 668 0.61 19 C40.07 0.32 2.11 0.011 0.011 0.03 0.004 — — — — 1.1 416 707 0.59 19 C50.08 0.40 2.38 0.011 0.009 0.03 0.004 — — — — 0 477 898 0.53 21 D1 0.070.01 1.26 0.009 0.01  0.03 0.003 0.49 — — — 5.0 370 455 0.81 26 D2 0.080.01 1.60 0.01  0.013 0.04 0.005 0.3  — — — 3.0 358 486 0.74 28 D3 0.070.01 1.46 0.01  0.011 0.02 0.004 0.48 — — — 2.1 311 468 0.66 26 D4 0.080.73 1.41 0.01  0.01  0.03 0.005 0.56 — — — 1.7 327 570 0.57 25 E1 0.080.03 1.35 0.011 0.009 0.04 0.004 0.51 0.32 — — 2.5 341 471 0.73 27

TABLE 2 Steel C Si Mn P S Al N Cr Mo Ti B A_(Re) R_(eL) R_(m) R_(eL)/RA₈₀ strip [% by weight] [%] [N/mm²] [N/mm²] [—] [%] F1 0.08 0.04 1.50.013 0.014 0.06 0.01 0.52 — 0.029 0.0031 0 278 521 0.53 24

TABLE 3 Annealing Cooling Galvanising Belt Steel Preheater furnace zoneNozzle bath speed strip [° C.] [m/min] F1 830 870 480 325 460 70

TABLE 4 Steel C Si Mn P S Al N Cr Mo Ti B A_(Re) R_(eL) R_(m) R_(eL)/RA₈₀ strip [% by weight] [%] [N/mm²] [N/mm²] [—] [%] G1¹ 0.072 0.09 1.49— 0.01 0.103 0.0047 0.5 — — 0.0045 0 241 521 0.463 21.7 G1² ″ ″ ″ ″ ″ ″″ ″ ″ ″ ″ 0 295 563 0.524 15.0 G1³ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ 0.9 264 4880.541 27.8 G1⁴ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ ″ 0 267 515 0.518 23.1

TABLE 5 Annealing Holding Holding temperature period Overageing periodSteel Type [° C.] [s] [° C.] [S] G1¹ Continuous 780 75 350 180 annealingG1² Continuous 800 75 350 180 annealing G1³ Hot 780 75 460  60dipgalvanising G1⁴ Hot 800 75 460  60 dipgalvanising

What is claimed is:
 1. A higher strength steel strip or steel sheetcomprising a predominantly ferritic-martensitic microstructure with amartensite content of between 40% and 20%, wherein the steel strip orsteel sheet, apart from Fe and impurities due to smelting, comprises (in% by weight) C: 0.05-0.2%; Si: ≦1.0; Mn: 0.8-2.0% P: ≦0.1%; S: ≦0.015%;Al 0.02-0.4%; N: ≦0.005%; Cr: 0.25-1.0%; B: 0.002-0.01%; and

optionally Ti: wherein for A1 contents of 0.02-0.06%, the Ti content isat least 2.8×A_(N), with A_(N)=content of N; and wherein the A1 contentis 0.1-0.4% if there is no Ti present.
 2. The steel strip or steel sheetaccording to claim 1, characterised in that its B content is 0.002 to0.005% by weight.
 3. A method for producing a steel strip or steel sheetaccording to claim 1 in which the steel strip or steel sheet is producedby cold rolling a hot strip, characterised in that the cold-rolled steelstrip or steel sheet is subjected to an annealing treatment in acontinuous furnace during which treatment the annealing temperature isbetween 750° C. and 870° C., and in that the annealed steel strip orsteel sheet is subsequently cooled down from the annealing temperatureat a cooling rate of at least 20° C./s and at most 100° C./s.
 4. Amethod for producing a steel strip or steel sheet according to claim 1in which the steel strip or steel sheet is produced by annealing a thinhot strip, characterised in that the steel strip or steel sheet as athin hot strip is subjected to an annealing treatment in a continuousfurnace during which treatment the annealing temperature is between 750°C. and 870° C., and in that the annealed steel strip or steel sheet issubsequently cooled down from the annealing temperature at a coolingrate of at least 20° C./s and at most 100° C./s.
 5. The method accordingto claim 3, characterised in that following cooling, the continuouslyannealed, cooled steel strip or steel sheet is subjected to anoverageing treatment in an overageing zone.
 6. The method according toclaim 5, characterised in that the holding period in the overageing zoneis up to 300 s and a treatment temperature of 300° C. to 400° C.
 7. Themethod according to claim 5, characterised in that following overageing,the steel strip or steel sheet is subjected to hot dip refining.
 8. Themethod according to claim 7, characterised in that the hot dip refiningstep comprises a galvanizing treatment, and the treatment durationrequired for galvanizing and passing through the overageing zone is upto 80 s, and the treatment temperature is between 420° C. and 480° C. 9.The method according to claim 8, characterised in that aftergalvanizing, a galvannealing treatment is carried out.
 10. The methodaccording to claim 3, characterised in that the steel strip or steelsheet is subsequently dressed as a final step of the method.
 11. Themethod according to claim 3 wherein the annealing temperature is between750° C. and 850° C.
 12. The method according to claim 4 wherein theannealing temperature is between 750° C. and 850° C.